The production of sodium tripolyphosphate (Na.sub.5 P.sub.3 O.sub.10) from wet-process phosphoric acid (H.sub.3 PO.sub.4) suitable for industrial uses requires removal of impurities such as iron (Fe), aluminum (Al), magnesium (Mg), calcium (Ca), fluorine (F), and sulfuric acid (H.sub.2 SO.sub.4). The removal of these impurities represents a major technical problem. Several published approaches are summarized below.
In the most common practice, impurities of wet-process phosphoric acid (30 percent P.sub.2 O.sub.5) are precipitated to a varying extent by neutralization with either sodium hydroxide (NaOH) or sodium carbonate (Na.sub.2 CO.sub.3).
In another method, wet-process phosphoric acid (30 percent P.sub.2 O.sub.5) is neutralized with ammonia (NH.sub.3) to the level of monoammonium phosphate (NH.sub.4 H.sub.2 PO.sub.4), thereby precipitating most of the impurities. After separating the precipitate, the solution is treated with sodium hydroxide to crystallize sodium ammonium phosphate (NaNH.sub.4 HPO.sub.4). After their separation, these crystals are dissolved by heating with sodium hydroxide solution at such a ratio that a crystal mixture of NaH.sub.2 PO.sub.4 + 2Na.sub.2 HPO.sub.4 results.
In a further method, (U.S.P. Pat. No. 3,574,535) wet-process phosphoric acid (30 percent P.sub.2 O.sub.5) is extracted with n-butanol. The organic phase, consisting of n-butanol, phosphoric acid, water, and some extracted impurities are neutralized with a sodium carbonate or sodium hydroxide solution to obtain a mixture of one mole of monosodium phosphate (NaH.sub.2 PO.sub.4) per two moles of disodium phosphate (Na.sub.2 HPO.sub.4). The crystal suspension which contains that portion of the impurities originally extracted to the butanol phase is separated from the latter by decantation, dried, and calcined.
Impure sodium tripolyphosphate manufactured from wet-process phosphoric acid is purified either by recrystallizing the sodium tripolyphosphate or by washing the sodium tripolyphosphate with a small amount of water to dissolve the impurities together with a portion of the sodium tripolyphosphate (U.S.P. Pat. No. 3,391,990).
Impurity removal from relatively dilute wet-process phosphoric acid by neutralization or by n-butanol extraction alone usually does not furnish a sodium tripolyphosphate product suitable for many applications requiring high purity. Recrystallization either at the sodium phosphate stage or at the sodium tripolyphosphate stage improves the purity of the product at the expense of lower phosphate recovery.